This invention relates generally to voltage controlled oscillators and more particularly to voltage controlled oscillators having an automatic gain control circuit.
Those skilled in the art will appreciate that frequency synthesizers have become the preferred means of frequency generation in most radio transceiver applications. The heart of a frequency synthesizer is the voltage controlled oscillator (VCO). Of course, the basic operation of a VCO, that is, to provide a changing frequency depending upon an applied voltage (commonly referred to as a steering line voltage), is widely known. Historically, early VCO's had very limiteu tuning ranges or bandwidths. As technology advanced, wider bandwidth VCO's became available and the trend today is to provide even broader bandwidths.
A significant problem encountered when trying to increase the tuning range for a VCO is varactor diode rectification at low steering line voltages. As is well known in the art, the frequency of a VCO is varied by changing the capacitance of one or more varactor diodes, which in turn changes the resonant frequency of a resonant circuit formed by the varactors and associated inductive and capacitive elements. However, to be effective, the peak radio frequency (RF) voltage across the varactor diodes should be less than the rectification level under all DC bias conditions (including temperature and transient effects). If the peak RF voltage across the varactors becomes greater than the DC reverse bias level, the varactors will become forward biased for a portion of the RF waveform cycle leading to rectification spurs and a degradation in sideband noise performance of the VCO.
To correct this problem, some VCO's have employed automatic gain control (AGC) circuits wherein the output level of the VCO is held constant. This is generally accomplished by detecting the output level of the VCO and providing a feedback signal to adjust the gain of the oscillator. Once the output level is established, the minimum steering line voltage can be set greater than the peak RF voltage level to prevent rectification. While this technique generally provides satisfactory VCO performance, the limited output level requires subsequent amplification (in a transceiver realization) to provide sufficient power to the mixers or other subsequent circuitry. Thus, limiting the output power of the VCO to improve rectification spur performance is adverse to the overall operation of the frequency synthesizer since the signal must be reamplified, and then relimited, to provide an acceptable signal to the mixer.